1. Field of the Invention
The present disclosure relates to an optical read/write apparatus that reads and writes information from/on an optical storage medium such as an optical tape, an optical disc or an optical card. More particularly, the present disclosure relates to an apparatus that carries out verification on data being written on an optical storage medium.
2. Description of the Related Art
Recently, the size of digital data to process has been rising steeply year by year as the resolutions of video data and still picture data have been tremendously increased and as increasing numbers of paper media have been converted into electronic ones. Meanwhile, so-called “crowd computing” technologies that allow people to use various kinds of applications and services via servers and storage systems on some network have become more and more popular nowadays. According to such crowd computing technologies, as a huge number of users save various kinds of data on that storage system on the network, the amount of data accumulated there should keep on skyrocketing from now on.
In the meantime, as regulations have been established one after another with regard to the duty of preserving such a huge amount of data saved, it should also be increasingly important to devise a method for saving that enormous amount of data as securely and as reliably as possible.
An apparatus that writes data of such a huge size optically on a storage medium must perform the operation of seeing if signal marks have been recorded just as intended on the optical storage medium in order to increase the reliability of writing. Such an operation will be referred to herein as a “verify operation”.
A known read/write apparatus that reads and writes data from/on an optical storage medium such as a write-once or rewritable optical disc reads data that has been just written and compares the data that has been read to the data to write in order to detect an error, if any, lest the write operation should fail due to a defect on the storage medium.
Such a verify operation is often performed in a unit at which a constant write or transfer rate can be maintained, not after everything has been written. That is to say, every time the disc has turned to a predetermined degree, the write operation is suspended, a track jump is made to return to the previous location, that portion on which data has just been written is scanned to detect any error, and then a track jump is made once again to move to a different area and write the next data there. And this series of operations is carried out over and over again by the read/write apparatus. That is why although reliability can be certainly ensured in this way for the data that has been written, it takes a longer time to get the write operation done.
If any error is detected when the data that has just been written is read, then the write operation is retried on another area, not the area on which the write error has occurred. On an optical disc, a set of data and its ID information are stored on the basis of a unit area called a “sector”. Thus, the data that has been written with an error on a sector is rewritten on another sector (which will be referred to herein as a “replacement sector”). A known read/write apparatus that makes such data correction is disclosed in Japanese Laid-Open Patent Publication No. 59-113509 (which will be referred to herein as “Patent Document No. 1” for convenience sake), for example.
Lately, as candidate read/write apparatuses that can save and archive data for a long time in order to meet the rising demand for storing a huge size of data with as high reliability as possible, proposed are an apparatus that uses a so-called “optical tape”, which is a kind of an optical storage medium in a tape shape, and an apparatus that handles a combination of multiple optical disc drives at the same time. Such a read/write apparatus to process a huge size of data should not only write and transfer data at sufficiently high rates but also keep the reliability of the written data as high as possible.
Nevertheless, for a storage medium with a low degree of random accessibility such as the tape medium, it is difficult to increase the write rate as long as such a method of writing data and checking the data just written time-sequentially over and over again is adopted as in the known optical disc drive described above.
Thus, to meet such a demand, a so-called “DRAW (direct read after write)” technique for performing a write operation and a read operation for verification purposes at the same time has been proposed. A known read/write apparatus that adopts such a DRAW technique is disclosed in Japanese Laid-Open Patent Publication No. 63-249941 (which will be referred to herein as “Patent Document No. 2” for convenience sake), for example.
The apparatus disclosed in Patent Document No. 2 makes a diffraction grating split the light that has been emitted from a light source into a write beam and two read beams and irradiates the same track on a storage medium with those three beams. As a result, the write beam forms a light beam spot as a write beam spot on one track and the two read beams form two light beam spots as read beam spots on the same track, thereby getting the DRAW operation done (i.e., writing data and reading the data that has just been written in parallel). By adopting such a technique, even an apparatus that processes a storage medium with big storage capacity but a low degree of random accessibility such as an optical tape or an apparatus including multiple optical disc drives in combination can also perform the verify operation while continuing the write operation. Consequently, a system that achieves even higher write and transfer rate and ensures a good deal of reliability is realized.
To stabilize the read operation to be performed by such a read/write apparatus using a read beam spot formed by the light that has been emitted from one light source while writing data with a write beam spot left by the same light source, Japanese Laid-Open Patent Publication No. 5-73913 (which will be referred to herein as “Patent Document No. 3” for convenience sake) proposes carrying out an automatic gain control (AGC) on the output read signal based on a write signal supplied from an LD modulator. Likewise, Japanese Laid-Open Patent Publication No. 8-249660 (which will be referred to herein as “Patent Document No. 4” for convenience sake) proposes switching the gain of the output read signal based on a write beam spot modulation signal supplied from a microprocessor unit (MPU).
Furthermore, Japanese Laid-Open Patent Publication No. 8-255345 (which will be referred to herein as “Patent Document No. 5” for convenience sake) proposes an apparatus that performs a DRAW operation by using reflected light beams that have left two read beam spots before and after a write beam spot on a track in order to deal with a variation in the level of a verify signal due to a defect on a given storage medium, for example. That apparatus includes a circuit that performs either division or subtraction on a signal obtained by getting a signal, from which a write modulated component has been removed by changing the gain of a read signal representing the reflected light that has left a first read beam spot moving ahead of the write beam spot, delayed for a predetermined amount of time by a delay circuit and on a signal from which a write modulated component has been removed by changing the gain of a read signal representing the reflected light that has left a second read beam spot moving behind the write beam spot. Patent Document No. 5 proposes minimizing the influence of any defect on a storage medium by performing the verify operation based on the output of such a circuit. And Patent Document No. 5 also proposes that the delay time in such a situation be set to be d/v, where d is the interval between the two read beam spots and v is the relative velocity of a light beam spot with respect to the storage medium.
According to the technique disclosed in Patent Document No. 2, a read signal is obtained by calculating the difference between the two signals representing the reflected light beams that have left the two read beam spots. On the other hand, according to the techniques disclosed in Patent Document Nos. 3 to 5, a read signal is obtained by dividing a signal representing the reflected light that has left the read beam spot moving behind the write beam spot by the write modulated signal to be input to the light source (i.e., by changing the gain). With any of these techniques, however, only a read signal of poor signal quality can be obtained and it is difficult to get the verify operation done accurately as will be described later.
Also, according to these known techniques, if any operating environment parameter such as the output power or the temperature changes, then the time delay between two signals to compare also changes, thus making it impossible to perform the comparison and correlation operations with good stability.
Furthermore, if the DRAW technique is applied to a storage medium with multiple information storage layers, the quality of the read signal will be debased due to the adverse influence of light that has been reflected from a storage layer other than the target one, from/on which data needs to be read or written.
Thus, an embodiment of the present invention provides an optical pickup and optical read/write apparatus that can obtain a good read signal by removing such a write modulated component from the read signal.
Another embodiment of the present invention provides an optical pickup and optical read/write apparatus that can get the comparison and correlation operations done with stability by obtaining a good predicted waveform even if the time delay between a signal representing the write modulated component and a signal representing the intensity of the read beam has varied.
Still another embodiment of the present invention provides an optical pickup and optical read/write apparatus that can reduce the adverse influence of such light that has been reflected from a non-target storage layer when the DRAW technique is applied to a storage medium with multiple storage layers.